Wavelength division multiplexing (WDM) enables different wavelengths to be simultaneously carried over a common fiber optic waveguide. Each wavelength or light beam carries encoded data. WDM can separate the fiber bandwidth into multiple discrete channels with narrow channel spacing through a technique referred to as dense wavelength division multiplexing (DWDM). This technique provides a relatively low-cost method of substantially increasing long haul telecommunication capacity over existing fiber optic transmission lines.
Techniques and devices are required for multiplexing the discrete wavelengths in DWDM transmission systems. In other words, the individual optical signals must be combined onto a common fiber optic waveguide. Then, the optical signals must be separated again into the individual signals or channels at the opposite end of the fiber optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength bands) from a broad spectral source is of significant importance to the fiber optic telecommunications field. Similarly, this technique is important in many other fields employing optical networking devices.
Devices that couple multiple, closely-spaced carrier wavelengths within a single optical fiber are called multiplexers. Devices that separate the carrier wavelengths at the receiving end of a fiber are called demultiplexers or channelizers.
As fiber optic transmissions enter and leave metropolitan and local area networks (LANs), each data-carrying wavelength is usually switched through various points along the fiber optic network. These points are known as “nodes.” At node locations, optical signals can be forwarded to the next node or “dropped” towards their final destination via the best possible path. The best possible path may be determined by such factors as distance, cost, and the reliability of specific routes. In addition, specific data-carrying wavelengths may be recombined or “added” to the multiplexed optical signal at node sites. The devices that perform these functions in DWDM network systems are called add/drop multiplexers (ADMs).
The conventional way to drop a data signal from a DWDM fiber is to de-multiplex the signal into its constituent wavelengths. Next, the light is detected using a photodetector, thus converting the signals to an electronic form (OE conversion). The electronic signal is switched and/or routed, as appropriate. The remaining signals are converted back to an optical signal (EO conversion). The optical signal is then sent down the proper fiber. During this last step, a signal can be added to the remaining signals. Such OE and EO conversion operations are both protocol and data rate dependent. These operations also require inflexible devices that are costly and difficult to upgrade as system capacity demand is increased.
Optical add/drop multiplexers (OADMs) have several significant advantages. First, OADMs cost less because they eliminate the need for much of the expensive high-speed electronics in conventional devices. Second, OADMs require smaller packaging because removing the electrical conversion step results in a reduced component count within the switches. Finally, optical devices are relatively future-proof because the optics can accommodate any bit-rate, whereas electrical devices must always be customized for the bit-rate and protocol of the signals.
Optical add/drop systems are comprised of two major subsystems. The first subsystem is the demultiplexing and multiplexing subsystem for selecting and recombining the appropriate wavelength. The second subsystem is the add/drop apparatus for routing the wavelength to the desired optical fiber output. Existing techniques for wavelength separation from a multiplexed signal using optical architectures include thin film bandpass filters, Fabry-Perot filters, fiber Bragg or diffraction grating filters, and polarization controllers. Each of these optical filtering methods may have different forms.
Thin film bandpass filters have traditionally been used in OADM devices to select single wavelengths from a multi-channel optical signal. Although such filters have good channel isolation, they tend to exhibit a transmission light loss of approximately 10%. Such filters are also highly temperature-sensitive. Further, they often operate in only one direction. In addition, such filters are limited to a single, fixed wavelength. Thus, to construct a multi-channel OADM device, multiple filters must be combined. This results in increased complexity, optical loss, and cost.
In U.S. Pat. No. 5,751,456, Koonen disclosed an example of a solution to some of these issues wherein a narrow-bandpass Fabry-Perot filter was utilized in a bidirectional OADM. As Fabry-Perot filters can have a bandpass of 1–2 nm or less, they can provide better isolation and lower loss factors than other thin film interference filters. FIG. 1 illustrates an example of the Koonen prior art. The device illustrated in FIG. 1 is limited in that it can add/drop only a single wavelength. As illustrated in this example, a circulator 10 is used to pass four wavelengths λ1–λ4 to a Fabry-Perot filter 11. Filter 11 selects one wavelength λ1 for continuation on to a receiver 12. The remaining wavelengths λ2–λ4 are reflected by the filter 11 back to a circulator 10. A transmitter 13 sends a new wavelength λ1′ to the filter 11. The new wavelength λ1′ is multiplexed with the original wavelengths λ2–λ4. The resulting wavelength is returned to the circulator 10 for continuation
The issue of such interference filter-based ADM devices being fixed in nature has been addressed in the prior art with the invention of “tunable” filters. Tunable filters can be selectively tuned to different wavelengths within a multi-channel optical signal. However, tuning thin-film optical filters requires that either the incident optical beam be repositioned with respect to the filter surface or that the filter itself be repositioned with respect to the input beam. Both scenarios require mechanical movement of components. These components include as actuators or stepper motors. The mechanical movement of these components makes these OADM devices active in nature. This results in increased complexity and cost.
FIG. 2 illustrates an example of a prior art tunable filter as disclosed in U.S. Pat. No. 6,292,299. FIG. 2 illustrates the mechanical nature of selecting a single wavelength. FIG. 2 also illustrates the potential complexity of matching the add/drop wavelengths to output fibers. An electronic controller 6 directs an optical filter 1 to move in the x and z directions to a specific location where a single wavelength from an incoming fiber 3 is intercepted. Once selected, the wavelength is passed or dropped to a fiber 2. The unselected wavelengths are reflected to continue on a fiber 4. A wavelength can be added from fiber 2 at the same time. As can be seen from the example illustrated in FIG. 2, the electronic controller must be mechanically manipulated to select a single wavelength.
Diffraction gratings and fiber Bragg grating filters (FBGs) offer alternative means of selecting and isolating single wavelengths from a multi-channel input beam in OADM devices. Diffraction gratings can be used in an OADM device to separate an input beam into its components in one direction, and recombine the wavelengths in the reverse direction. However, with diffraction grating systems, the component count can rise rapidly. Lenses, collimators, and focusing optics are required to refine, direct, and couple the light beams into fibers.
Because FBGs are constructed from optical fibers, rather than individual thin-film filter substrates, they allow for all-fiber systems to be constructed. Fiber Bragg grating systems offer high levels of selectivity. However, they are limited in that several fiber gratings must be combined, along with optical circulators, in order to handle a multiplexed optical signal with a high channel count. The result can be a very large device with a high component count, increased complexity, and a higher cost. In addition, the combination or cascading of multiple-fiber Bragg gratings can significantly reduce signal strength as the insertion loss of multiple devices is compounded throughout the system.
A recent development in the area of wavelength selectivity and separation of multiplexed optical signals has been the utilization of polarization controllers. As disclosed by U.S. Pat. No. 6,285,478, polarization-controlling elements can also be used within OADM devices to separate a multi-channel WDM input signal into odd and even channels. This is done, for example, by splitting the signal into its vertically and horizontally polarized components. When combined with birefringent beam displacing optics, the separated signals can then be directed to appropriate output paths. This method provides an add/drop device that can accommodate the high channel counts and narrow channel spacing of current DWDM networks, where channels are separated by 50 GHz or less. This channel-separation technique is expandable and can adapt to increasing channel counts. However, this technique is subject to very high optical component count. Included in the optical component count are multiple polarization controllers, birefringent elements and beam splitters. These components are required to manipulate dense multiplexed signals. Assembling and aligning these optical components within a device can be extremely expensive. This is particularly true when high levels of precision are required. FIG. 3 illustrates an example of the prior art. FIG. 3 illustrates an example of the large number of components required to separate eight-channels.
With the continued development of WDM fiber optic systems, it is becoming increasingly important to control the direction of wavelengths to desired output ports (i.e., routers). It is likewise important to permit a new signal to replace an existing signal at a specific wavelength (i.e., add/drop) using optical systems. Furthermore, since the development of DWDM sends hundreds and even thousands of wavelengths through a fiber, the ability to selectively control a single or several wavelengths without affecting the other wavelengths is very important. This ability is important because the optical to electrical to optical conversion process is expensive and uses significant power as well as space. In particular, optical add/drops are critical components in WDM regional-access ring or bus networks to provide broadband access to users.
Current optical subsystems that perform add/drop functions include mirrors and micro-electro-mechanical systems (MEMS) using movable and fixed mirrors and etalons.
In the prior art, it is known to use a reconfigurable switching matrix having front and back micromirrors. These micromirrors are a reconfigurable switching matrix capable of directing the output of wavelengths in multiple directions.
It is also known in the art to use a tunable optical add/drop that employs an optical filter device, such as a multi-layer dielectric wedge filter. This technique is successful using tunable Mach-Zehnder interferometers, acoustic tuning filters, tunable thin film interference filters, tunable Fabry-Perot etalons, and tunable Fabry-Perot interferometers. However, it is only possible to interact with a single wavelength at one time using this technique.
A wedged etalon with an actuator that moves the etalon to the position of the channel to be added or dropped may also be employed. However, this system can only accommodate adding or dropping a single channel simultaneously. Further, the added or dropped channel must be at the same frequency.
Accordingly, in light of the limitations of the prior art, it is desirable to have an optical add/drop system which is simpler then those known in the art, has low optical loss characteristics, operates on single or multiple channels, and is capable of adding or dropping finely-spaced channels with separations as close as 50 MHz.